The present invention relates to a process and apparatus for selective catalyzed NO.sub.x -reduction in oxygen-containing exhaust gases, in which, for improving the reduction of nitric oxides, hydrocarbons and air are supplied on the upstream side of an exhaust gas purifier.
In the future, extremely strict nitric oxide limit values will apply to diesel internal-combustion engines in addition to very low particle limit values. It will be extremely difficult to maintain these extremely strict nitric oxide limit values only by measures within the engine.
It is known that, in the case of conventional selective catalyzed reduction processes, ammonia is used as a selective reducing agent for nitric oxides. As an alternative, aqueous urea solutions have also been suggested which, however, just like ammonia, because of an ammonia slip, may result in problems in the case of a transient engine operation.
In the case of vehicles, solutions which use hydrocarbons for the post-engine nitric oxide reduction are deemed more advantageous.
DE 42 21 363 A1 describes an exhaust gas purification process arrangement which comprises a feeding system for liquid hydrocarbons made available in a liquid-hydrocarbon storage tank as well as a feeding system for feeding compressed air. A spray-in device, which is situated in the flow path of the exhaust gas upstream of an exhaust gas purifier, sprays in the liquid hydrocarbon after the guiding-together with the compressed air into the flow of the exhaust gas.
An object of the present invent-ion is to improve the aforementioned process and simplify an arrangement for implementing that process without any impairment of a maximally achievable NO.sub.x -conversion in the NO.sub.x -exhaust gas purifier.
This object has been achieved according to the present invention by providing that the metering pipe and an air pipe are connected to the cracked catalyst controllable as function of temperature, and the catalytically cracked defined fuel quantity is oxidizable by simultaneously supplied air, and a supply pipe leading into the exhaust pipe for supplying resulting reactive species being to exhaust gas flow upstream of the exhaust gas purifier and the cracked catalyst is configured to be electrically heatable and controllable in a temperature range between 200.degree.-700.degree. C.
By the arrangement of a cracked catalyst, via which a portion of the fuel provided for the fuel supply of the internal-combustion engine is guided, and by the simultaneous blowing of air into this cracked catalyst, highly effective hydrocarbons are produced which have a high NO.sub.x reduction potential.
Such measures have the advantage of using the fuel, which is carried in the vehicle anyhow, also for the NO.sub.x -reduction. Therefore, no additional tank is required for the storage of hydrocarbons.
In addition to the partial oxidation of the hydrocarbons, the air blown into the cracked catalyst also causes a periodically implemented purification of the coke-susceptible cracked-catalyst surface in a highly oxidizing atmosphere; for example, at .lambda.&gt;1 and a temperature in the Denox-catalyst which is higher than 500.degree. C.
The use of a pressure accumulator in the feeding pipe, in which gaseous hydrocarbons are stored, according to the need and requirements, permits a metered addition of gaseous hydrocarbons into the exhaust gas flow of the internal-combustion engine for the optimal NO.sub.x -reduction in the exhaust gas purifier.